Franz Kafka's The Metamorphosis is composed of layer upon layer of symbolic meanings. Suffice it to bear in mind that the original German title -Die Verwandlung- admits of a number of translations, such as "change", "mutation", "conversion", and that it seems to be understood as "metamorphosis" only when associated with Greek mythology.

Why is this relevant to your question? Let's see. The three boarders behave as if they were a single character, although they follow the lead of "the middle lodger". During the Dorian (indo-European) invasion of Greece, that went under a different name at the time, a trinity of male gods under the leadership of one (Zeus) superseded the original female trinity -or three aspects of the Mother Goddess. Once the three boarders have settled down at the Samsa's, the three functional members of the family do their bid. They have, symbolically, been dispossessed of authority in their own home.

Gregor has been kept out of their sight, but one day he peeps out of his bedroom door, lured by the music played in the sitting room. The boarders are outraged by this disgusting creature and threaten to leave without paying their rent.

After this unsavory scene, Gregor dies in the small hours of the morning. His demise seems to bring his father back to his senses. He forces the boarders to look at Gregor's body and then throws them out of the house, thus regaining his authority and virility, for indeed it appears that the boarders have -again, symbolically- emasculated him.

Other interpretations claim that the boarders provide comic relief in an increasingly anguishing story. This could be justified by the following paragraph:

The gentlemen bent over the dishes set in front of them as if they wanted to test the food before eating it, and the gentleman in the middle, who seemed to count as an authority for the other two, did indeed cut off a piece of meat while it was still in its dish, clearly wishing to establish whether it was sufficiently cooked or whether it should be sent back to the kitchen.

In the multiple meanings that one could ascribe the boarders, an interesting one is that they stand for the harsh outside world, a world that will mercilessly crush the weak. That Kafka intended them to be symbols rather than characters is clear from the fact that he did not give them names. Moreover, the family's subservience to these one-in-three strangers for money may imply a warning against the lengths to which humans are prepared to go when moral values yield to selfish rationalizations.